As shown in FIGS. 9(a) to 9(c) and 10(a) to 10(c), an injection molder stacks two metal molds 1 and 2, and supplies a raw resin (resin material) 3 into a space formed between the metal molds 1 and 2 to mold a resin product having a desired shape. To this end, the injection molder includes a mold closing apparatus 10 for stacking the two metal molds 1 and 2 and an injection apparatus 20 for injecting and supplying the raw resin 3 into the space formed between the metal molds 1 and 2. It is to be noted here that the metal mold 1 is formed in a concave shape, and the metal mold 2 is formed in a convex shape.
The mold closing apparatus 10 includes a fixed die plate 11 fixed on a base 31 on which the metal mold 1 is mounted, a movable die plate 12 on which the other metal mold 2 is mounted and which can move toward and away from the fixed die plate 11, a tie bar 14 having one end connected to the fixed die plate 11 and the other end connected to a connecting plate 13 for guiding movement of the movable die plate 12, and a boost cylinder 15 for driving the movable die plate 12.
Consequently, if the boost cylinder 15 is operated, then the metal mold 2 is moved toward and away from the metal mold 1 together with the movable die plate 12 under the guidance of the tie bar 14. When the metal molds 1 and 2 are stacked, the boost cylinder 15 presses the metal mold 2 to the metal mold 1 to oppose the pressure of the resin into the space between the metal molds 1 and 2.
The movable die plate 12 includes an extrusion cylinder 16 for taking out a resin product 4 fitted on the convex metal mold 2 side after molding.
Meanwhile, the injection apparatus 20 includes an injection cylinder 22 having a nozzle 21 provided at a tip thereof, an injection screw 23 inserted for back and forth movement and for rotation in the injection cylinder 22, a heater 24 for heating the raw resin 3 in the injection cylinder 22, a table (driving apparatus table) 25 fixed on a base 32, a fixed frame 26 fixed on the table 25 for supporting the injection cylinder 22, a movable frame 27 which is mounted for movement on the table 25 and to which the injection screw 23 is connected, a screw rotating motor 28 mounted on the movable frame 27 for driving the injection screw 23 to rotate, an injection screw moving mechanism 29 mounted between the fixed frame 26 and the movable frame 27 for moving the injection screw 23 together with the movable frame 27 in an axial direction, and a hopper 30 for supplying the raw resin 3 into the injection cylinder 22.
It is to be noted that a nozzle forward and backward moving cylinder 33 for moving the table 25 in an axial direction of the injection cylinder 22 to adjust the position of the nozzle 21 forwardly or backwardly is provided between the table 25 and the fixed die plate 11.
Consequently, it is possible, by rotating the injection screw 23 by means of the screw rotating motor 28, to introduce the raw resin 3 in the form of pellets from the hopper 30 into the injection cylinder 22 and feed the raw resin 3 forwardly. Thereupon, the raw resin 3 can be melted by heating the raw resin 3 by means of the heater 24. Then, the injection screw 23 is advanced by operating the injection screw moving mechanism 29, and as a result, the molten resin accumulated in the nozzle 21 can be injected into the space between the metal molds 1 and 2.
In the injection molding apparatus described above, the injection molding is performed in such a manner as shown in FIGS. 9(a) to 9(c), 10(a) to 10(c) and 11.
In particular, the movable die plate 12 is first set to such an original position as shown in FIG. 9(a) (an original position setting step, step S1 of FIG. 11). Then, the movable die plate 12 is moved as shown in FIG. 9(b) to stack the metal molds 1 and 2 to perform mold closing (a mold closing step, step S2 of FIG. 11).
Then, the molten resin for one shot is accumulated in the nozzle 21. Then, as shown in FIG. 9(c), the injection screw 23 is advanced by operating the injection screw moving mechanism 29 so that the molten resin is injected into the space between the metal molds 1 and 2 while the molten resin is pressurized (a pressurization and injection step, step S3 of FIG. 11)
After the molten resin is injected into the space between the metal molds 1 and 2, the pressurized pressure state is kept for a predetermined period of time using a timer or the like. Since the resin shrinks to decrease its volume when the resin solidifies, the injection screw 23 is advanced to replenish the resin for the shrinkage (dwelling step, step S4 of FIG. 11) as shown in FIG. 10(a). Thereafter, the molded product is cooled, and the screw rotating motor 28 and the injection screw moving mechanism 29 are operated to keep the molten resin pressure to a certain pressure. In this state, the raw resin 3 in the form of pellets thrown in the hopper 30 is melted and fed by rotating the injection screw 23 to accumulate the resin for one shot in the nozzle (cooling and metering step, step S5).
After the cooling and metering step is completed (step S6 of FIG. 11), the movable die plate 12 is moved to separate the metal molds 1 and 2 away from each other to perform mold opening (mold opening step, step S7 of FIG. 11) as shown in FIG. 10(b). Then, as shown in FIG. 10(c), the extrusion cylinder 16 is operated to extrude the resin product 4 fitted on the side of the convex metal mold 2 (product extruding step, step S8 of FIG. 11). Thereafter, the steps from the mold closing step (step S2 of FIG. 11) to the product extruding step (step S8 of FIG. 11) are carried out again until a molding completion decision (step S9 of FIG. 11) is made.
It is to be noted that, while the injection molding is performed in such a manner as described above, as an injection molder, there is an electric injection molder which uses an electric motor for the injection screw moving mechanism 29 in addition to a hydraulic injection molder which uses a fluid pressure cylinder such as a hydraulic cylinder for the injection screw moving mechanism 29. The electric injection molder can control, when compared with the hydraulic injection molder, the back and forth movement of the injection screw in a high degree of accuracy and in various forms.
Thus, when injection molding is performed in such a manner as described above by the electric injection molder, in the pressurization and injection step (step S3 of FIG. 11) for filling the resin into the metal mold, since the resin becomes solidified if the temperature of the resin drops during filling, the velocity of the injection screw is controlled upon such resin filling to prevent the solidification of the resin. Further, in the dwelling step and the cooling and metering step (steps S4 and S5 of FIG. 11) after the resin filling, the pressure of the injection screw is controlled for dwelling in order to correct the shrinkage occurring by the cooling of the resin is controlled.
In the resin filling step (pressurization and injection step), the electric motor for driving the injection screw to move back and forth is controlled so that the moving velocity in the forward and backward direction of the injection screw may be equal to a target velocity. Further, in the dwelling step and so forth after the resin filling, the electric motor for driving the injection screw to move back and forth is controlled so that the resin pressure by the injection screw maybe equal to a target pressure.
Therefore, it is necessary in the molding step to change over the driving control of the injection screw between the two control modes of the velocity control and the pressure control. Conventionally, the changeover (called V/P changeover) between the velocity control and the pressure control is performed in accordance with the position of the injection screw. Then, the position of the injection screw used as a changeover reference in the case described above is set based on an operation step when a skilled operator performs the injection molding several times by manual operation until high-quality molding is achieved successfully.
For example, FIG. 12 is a block diagram showing a conventional control apparatus for controlling the forward and backward movement of the injection screw upon injection molding.
As shown in FIG. 12, in the present control apparatus, the velocity control is performed in such a manner as described below. In particular, a deviation (Xr−X) between a position instruction value Xr which is a target value of the motor position (rotational angle) and a position detection value X which is a found value of the motor position (rotational angle) is calculated (adder 71), and the deviation (Xr−X) is multiplied by a position proportion gain Kp by a gain processing section 72 to convert it into a velocity-corresponding instruction value [Vr=Kp(Xr−X)]. Then, a deviation (Vr−V) between the velocity instruction value Vr and a velocity detection value V which is a found value of the motor velocity (rotational velocity) is calculated (adder 73), and a motor instruction value (current instruction value) is set based on the deviation (Vr−V) by a velocity control section 74 and outputted.
Further, in the control apparatus, the pressure control is performed in such a manner as described below. In particular, pressure instruction values P1 and P2 which are target values of the pressure (resin pressure) in the space between the metal molds are set. The pressure instruction value P1 is utilized for regulating so that the pressure in the space between the metal molds does not excessively rise upon the velocity control and for performing changeover from the velocity control to the pressure control. The other pressure instruction value P2 is utilized upon the pressure control.
Thus, in a changeover section 75, based on a V/P changeover signal, the pressure instruction value P1 is selected upon the velocity control (upon V control), but the pressure instruction P2 is selected upon the pressure control (upon P control). A deviation (Pr−P) between the selected pressure instruction value P1 or P2 (hereinafter referred to as Pr) and a pressure detection value P which is a found value of the pressure in the space between the metal molds is calculated (adder 76), and a motor instruction value (current instruction value) is set based on the deviation (Pr−P) by a pressure control section 77 and is outputted.
While selective changeover between the motor instruction value from the velocity control section 74 and the motor instruction value from the pressure control section 77 is performed by a changeover section 78, the changeover section 78 selects a motor instruction value having a lower value from between the motor instruction values just mentioned (low select). In a final stage of the velocity control, the viscosity of the raw resin increases because of temperature dropping as the position of the injection screw approaches a filling completion state, and the deviation (Xr−X) between the position instruction value Xr and the position detection value X becomes less liable to decrease. As a result, the actual velocity drops in a state wherein the velocity instruction value Vr does not decrease, and the motor instruction value from the velocity control section 74 increases.
At this time, the pressure instruction value P1 is selected, and the motor instruction value from the velocity control section 74 is higher than the motor instruction value from the pressure control section 77 which is based on the pressure instruction value P1, and as a result, the motor instruction value from the pressure control section 77 is selected. In other words, changeover from the velocity control to the pressure control is performed. Upon the pressure control, the pressure instruction value P2 is selected, and a motor instruction value from the pressure control section 77 which is based on the pressure instruction value P2 is inputted to the changeover section 78.
It is to be noted that the technique wherein the changeover between the velocity control and the pressure control is performed through the low select of the motor instruction value in such a manner as described above is disclosed in the official gazettes of Japanese Patent Laid-Open No. 369520/1992 and Japanese Patent No. 2660636.
Further, in the official gazette of Japanese Utility Model Publication No. 28253/1994, an apparatus is disclosed which performs automatic changeover among a velocity control circuit which controls the resin pressure by controlling the velocity of a servomotor upon dwelling, a position control circuit which controls the position and the velocity of the servomotor based on information from a position detection section, and a velocity control circuit which controls a velocity instruction value based on a deviation amount between a dwelling instruction signal upon dwelling and an actual dwell value outputted from a pressure sensor using a switching circuit.
Furthermore, in the official gazette of Japanese Patent Publication No. 67722/1995, a technique is disclosed wherein, when a screw actual thrust is higher than a preset value, a control signal for controlling the rotational velocity of an electric motor is outputted so that an actual thrust may be equal to the set thrust, but, when the screw actual thrust is lower than the preset value, another control signal for controlling the rotational velocity of the electric motor so that the screw may advance at the set velocity is outputted, and the rotational velocity of the electric motor is controlled in accordance with either one of the control signals described above and velocity and position feedback signals.
Meanwhile, also a technique is available wherein changeover between the velocity control and the pressure control (V/P changeover) is performed in response to the position of the injection screw. In this instance, the position of the injection screw used as a changeover reference is set based on an operation step when a skilled operator performs the injection molding several times by manual operation until high-quality molding is achieved successfully.
However, in the apparatus (shown in FIG. 12) which performs selection between the motor instruction value utilized for the velocity control and the motor instruction value utilized for the pressure control through the low select in such a manner as described above, the apparatus disclosed in the official gazette of Japanese Patent Utility Model Publication No. 28253/1994, and the apparatus disclosed in the official gazette of Japanese Patent Publication No. 67722/1995, it sometimes occurs that the changeover between the velocity control and the pressure control is performed frequently, and as a result, there is a disadvantage that the control is not stabilized.
In particular, in a situation wherein the control mode is changed over from the velocity control to the pressure, since the viscosity of the filled resin rises and the filled resin solidifies gradually, the state of the resin is very unstable, and therefore, the motor instruction value utilized for the velocity control and the motor instruction value utilized for the pressure control are not stabilized. Therefore, if the selection between the motor instruction value utilized for the velocity control and the motor instruction value utilized for the pressure control is performed through the low select in such a manner as described above, then the changeover between the velocity control and the pressure control is performed frequently, and as a result, the control is not stabilized. Consequently, not only the filling of the resin cannot be performed stably but also there is the possibility that some dispersion may occur in the quality of the molded products.
Further, in the technique wherein the changeover between the velocity control and the pressure control is performed in response to the position of the injection screw based on a result of performance by a skilled operator, if the V/P changeover is performed in response to the position of the injection screw set in advance, then the filling of the resin cannot be performed stably, and there is the possibility that dispersion may occur in the quality of the molded products.
In particular, the quality (shape, dimension and so forth) of the resin product by the injection molding is liable to be influenced by-the operation environment such as the temperature and the humidity. Therefore, if the V/P changeover is performed in response to a fixed position of the injection screw using the conventional velocity control or pressure control, then the quality of the resin product varies in response to variation of the operation environment, and the dispersion of the resin product increases.
It is to be noted that, as the technique relating to such changeover between the velocity control and the pressure control as described above, in the official gazette of Japanese Patent No. 2866361 and the official gazette of Japanese Patent No. 2921754, a technique is disclosed wherein the gain relating to the pressure feedback is set lower than that in normal operation immediately after the control mode is changed over to the pressure control to raise the stability of the pressure feedback. In this case, even if the control is stabilized, the control instruction value of the pressure feedback upon transition is not necessarily appropriate, and not only the filling of the resin cannot be performed suitably but also there is the possibility that some dispersion may occur in the quality of the molded products.
The present invention has been made in light of the subjects described above, and it is an object of the present invention to provide an electric injection molder and an injection molding method for an electric injection molder wherein, in injection molding by the electric injection molder, stable resin products having a minimized dispersion in the quality can be molded even if the operation environment varies.